Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, International Patent Application No. PCT/US95/15790.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in International Patent Application No. PCT/US97/02681 in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg—Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg—Ag-ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
The PCT/US95/15790 application disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. The PCT/US95/15790 application, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Because light is generated in organic materials from the decay of molecular excited states or excitons, understanding their properties and interactions is crucial to the design of efficient light emitting devices currently of significant interest due to their potential uses in displays, lasers, and other illumination applications. For example, if the symmetry of an exciton is different from that of the ground state, then the radiative relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric under exchange of spins of electrons comprising the exciton, the decay of a symmetric exciton breaks symmetry. Such excitons are known as triplets, the term reflecting the degeneracy of the state. For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (or singlet) exciton is created. (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S. R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Applied Physics Letters, 1999, 75, 4–6.) Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist for up to several seconds after excitation due to the low probability of the transition. In contrast, fluorescence originates in the rapid decay of a singlet exciton. Since this process occurs between states of like symmetry, it may be very efficient.
Many organic materials exhibit fluorescence from singlet excitons. However, only a very few have been identified which are also capable of efficient room temperature phosphorescence from triplets. Thus, in most fluorescent dyes, the energy contained in the triplet states is wasted. However, if the triplet excited state is perturbed, for example, through spin-orbit coupling (typically introduced by the presence of a heavy metal atom), then efficient phosphoresence is more likely. In this case, the triplet exciton assumes some singlet character and it has a higher probability of radiative decay to the ground state. Indeed, phosphorescent dyes with these properties have demonstrated high efficiency electroluminescence.
Only a few organic materials have been identified which show efficient room temperature phosphorescence from triplets. In contrast, many fluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang, “Recent developments in molecular organic electroluminescent materials,” Macromolecular Symposia, 1997, 125, 1–48; U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen, 1997)) and fluorescent efficiencies in solution approaching 100% are not uncommon. (C. H. Chen, 1997, op. cit.) Fluorescence is also not affected by triplet-triplet annihilation, which degrades phosphorescent emission at high excitation densities. (M. A. Baldo, et al., “High efficiency phosphorescent emission from organic electroluminescent devices,” Nature, 1998, 395, 151–154; M. A. Baldo, M. E. Thompson, and S. R. Forrest, “An analytic model of triplet-triplet annihilation in electrophosphorescent devices,” 1999). Consequently, fluorescent materials are suited to many electroluminescent applications, particularly passive matrix displays.
To understand the different embodiments of this invention, it is useful to discuss the underlying mechanistic theory of energy transfer. There are two mechanisms commonly discussed for the transfer of energy to an acceptor molecule. In the first mechanism of Dexter transport (D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys., 1953, 21, 836–850), the exciton may hop directly from one molecule to the next. This is a short-range process dependent on the overlap of molecular orbitals of neighboring molecules. It also preserves the symmetry of the donor and acceptor pair (E. Wigner and E. W. Wittmer, Uber die Struktur der zweiatomigen Molekelspektren nach der Quantenmechanik, Zeitschrift fur Physik, 1928, 51, 859–886; M. Klessinger and J. Michl, Excited states and photochemistry of organic molecules (VCH Publishers, New York, 1995)). Thus, the energy transfer of Eq. (1) is not possible via Dexter mechanism. In the second mechanism of Förster transfer (T. Förster, Zwischenmolekulare Energiewanderung and Fluoreszenz, Annalen der Physik, 1948, 2, 55–75; T. Förster, Fluoreszenz organisciler Verbindugen (Vandenhoek and Ruprecht, Gottinghen, 1951), the energy transfer of Eq. (1) is possible. In Förster transfer, similar to a transmitter and an antenna, dipoles on the donor and acceptor molecules couple and energy may be transferred. Dipoles are generated from allowed transitions in both donor and acceptor molecules. This typically restricts the Förster mechanism to transfers between singlet states.
Nevertheless, as long as the phosphor can emit light due to some perturbation of the state such as due to spin-orbit coupling introduced by a heavy metal atom, it may participate as the donor in Förster transfer. The efficiency of the process is determined by the luminescent efficiency of the phosphor (F Wilkinson, in Advances in Photochemistry (eds. W. A. Noyes, G. Hammond, and J. N. Pitts), pp. 241–268, John Wiley & Sons, New York, 1964), i.e., if a radiative transition is more probable than a non-radiative decay, then energy transfer will be efficient. Such triplet-singlet transfers were predicted by Förster (T. Förster, “Transfer mechanisms of electronic excitation,” Discussions of the Faraday Society, 1959, 27, 7–17) and confirmed by Ermolaev and Sveshnikova (V. L. Ermolaev and E. B. Sveshnikova, “Inductive-resonance transfer of energy from aromatic molecules in the triplet state,” Doklady Akademii Nauk SSSR, 1963, 149, 1295–1298), who detected the energy transfer using a range of phosphorescent donors and fluorescent acceptors in rigid media at 77K or 90K. Large transfer distances are observed; for example, with triphenylamine as the donor and chrysoidine as the acceptor, the interaction range is 52 Å.
The remaining condition for Förster transfer is that the absorption spectrum should overlap the emission spectrum of the donor assuming the energy levels between the excited and ground state molecular pair are in resonance. In one example of this application, we use the green phosphor fac tris(2-phenylpyridine)iridium (Ir(Ppy)3; M. A. Baldo, et al., Appl. Phys. Lett., 1999, 75, 4–6) and the red fluorescent dye [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl) ethenyl]-4H-pyran-ylidene]propane-dinitrile] (DCM2“; C. W. Tang, S. A. VanSlyke, and C. H. Chen, “Electroluminescence of doped organic films,” J. Appl. Phys., 1989, 65, 3610–3616). DCM2 absorbs in the green, and, depending on the local polarization field (V. Bulovic, et al., “Bright, saturated, red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts,” Chem. Phys. Lett., 1998, 287, 455–460), it emits at wavelengths between λ=570 nm and λ=650 nm.
It is possible to implement Förster energy transfer from a triplet state by doping a fluorescent guest into a phosphorescent host material. Unfortunately, such systems are affected by competitive energy transfer mechanisms that degrade the overall efficiency. In particular, the close proximity of the host and guest increase the likelihood of Dexter transfer between the host to the guest triplets. Once excitons reach the guest triplet state, they are effectively lost since these fluorescent dyes typically exhibit extremely inefficient phosphorescence.
To maximize the transfer of host triplets to fluorescent dye singlets, it is desirable to maximize Dexter transfer into the triplet state of the phosphor while also minimizing transfer into the triplet state of the fluorescent dye. Since the Dexter mechanism transfers energy between neighboring molecules, reducing the concentration of the fluorescent dye decreases the probability of triplet-triplet transfer to the dye. On the other hand, long range Förster transfer to the singlet state is unaffected. In contrast, transfer into the triplet state of the phosphor is necessary to harness host triplets, and may be improved by increasing the concentration of the phosphor.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes, a “hole transporting layer” (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an “electron transporting layer” (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
As noted above, light emission from OLEDs is typically via fluorescence or phosphorescence. There are issues with the use of phosphorescence. It has been noted that phosphorescent efficiency decreases rapidly at high current densities. It may be that long phosphorescent lifetimes cause saturation of emissive sites, and triplet-triplet annihilation may produce efficiency losses. Another difference between fluorescence and phosphorescence is that energy transfer of triplets from a conductive host to a luminescent guest molecule is typically slower than that of singlets; the long range dipole-dipole coupling (Förster transfer) which dominates energy transfer of singlets is (theoretically) forbidden for triplets by the principle of spin symmetry conservation. Thus, for triplets, energy transfer typically occurs by diffusion of excitons to neighboring molecules (Dexter transfer); significant overlap of donor and acceptor excitonic wavefunctions is critical to energy transfer. Another issue is that triplet diffusion lengths are typically long (e.g., >1400 Å) compared with typical singlet diffusion lengths of about 200 Å. Thus, if phosphorescent devices are to achieve their potential, device structures need to be optimized for triplet properties. In this invention, we exploit the property of long triplet diffusion lengths to improve external quantum efficiency.
Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence in certain electroluminescent materials. In contrast, only a small percentage of excitons in fluorescent devices, which are singlet-based, result in fluorescent luminescence.
An alternative is to use phosphorescence processes to improve the efficiency of fluorescence processes. Fluorescence is in principle 75% less efficient due to the three times higher number of symmetric excited states.
Because one typically has at least one electron transporting layer and at least one hole transporting layer, one has layers of different materials, forming a heterostructure. The materials that produce the electroluminescent emission are frequently the same materials that function either as the electron transporting layer or as the hole transporting layer. Such devices in which the electron transporting layer or the hole transporting layer also functions as the emissive layer are referred to as having a single heterostructure. Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure. The separate emissive layer may contain the emissive molecule doped into a host or the emissive layer may consist essentially of the emissive molecule.
That is, in addition to emissive materials that are present as the predominant component in the charge carrier layer, that is, either in the hole transporting layer or in the electron transporting layer, and that function both as the charge carrier material as well as the emissive material, the emissive material may be present in relatively low concentrations as a dopant in the charge carrier layer. Whenever a dopant is present, the predominant material in the charge carrier layer may be referred to as a host compound or as a receiving compound. Materials that are present as host and dopant are selected so as to have a high level of energy transfer from the host to the dopant material. In addition, these materials need to be capable of producing acceptable electrical properties for the OLED. Furthermore, such host and dopant materials are preferably capable of being incorporated into the OLED using starting materials that can be readily incorporated into the OLED by using convenient fabrication techniques, in particular, by using vacuum-deposition techniques.
The exciton blocking layer used in the devices of the present invention (and previously disclosed in U.S. appl. Ser. No. 09/153,144, now U.S. Pat. No. 6,097,147) substantially blocks the diffusion of excitons, thus substantially keeping the excitons within the emission layer to enhance device efficiency. The material of blocking layer of the present invention is characterized by an energy difference (“band gap”) between its lowest unoccupied molecular orbital (LUMO) and its highest occupied molecular orbital (HOMO). In accordance with the present invention, this band gap substantially prevents the diffusion of excitons through the blocking layer, yet has only a minimal effect on the turn-on voltage of a completed electroluminescent device. The band gap is thus preferably greater than the energy level of excitons produced in an emission layer, such that such excitons are not able to exist in the blocking layer. Specifically, the band gap of the blocking layer is at least as great as the difference in energy between the triplet state and the ground state of the host.
It is desirable for OLEDs to be fabricated using materials that provide electroluminescent emission in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colors, red, green and blue so that they may be used as a colored layer in an OLED or SOLED. It is also desirable that such compounds be capable of being readily deposited as a thin layer using vacuum deposition techniques so that they may be readily incorporated into an OLED that is prepared entirely from vacuum-deposited organic materials.
Co-pending application U.S. Ser. No. 08/774,087, filed Dec. 23, 1996, now U.S. Pat. No. 6,048,630, is directed to OLEDs containing emitting compounds that produce a saturated red emission.